Preheat the oven to 375ºF. Line a baking sheet with parchment paper and spray with non-stick cooking spray.
Place 1-inch of water in a small saucepan and place over medium heat until simmering. In a medium, heatproof bowl, add the butter, chocolate, sugar, dark brown sugar, and vanilla. Place on top of the simmering water and allow to melt. Stir until smooth. Remove and allow to cool to room temperature, about 5 minutes. Stir in egg whites.
In a medium bowl, add the flour, baking soda, cocoa powder and salt and whisk to combine. Add the flour mixture to the wet mixture. Stir until smooth. Pour batter onto the prepared baking sheet and spread to 1/4-inch thick. Sprinkle with sea salt.
Place in the oven and bake for 15 minutes until dry and crisp. Remove from the oven and allow to cool completely. Break the brownie brittle into pieces and serve.
Tip: Store in a zip-top bag for up to 1 week. Perfect as a snack dipped in milk!